It is well known to heat vehicle seats with a heating element comprising an electrically resistive conductor that generates heat upon an electric current being passed through it. Moreover, it is known to individually provide both the seat bottom portion and the seat backrest portion with such heating elements.
Early versions of such seat heating devices electrically connected the seat bottom portion and backrest portion heating elements in series, with both heaters controlled jointly by a single controller. Such devices suffered from occupants experiencing unpleasant physiological impressions of unequal heating between the seat bottom portion and backrest portion, despite occupant efforts to adjustably control the temperature setting via the device's control interface. Moreover, such devices tended to undesirably tax the vehicle's electrical system and required wiring harnesses having conductors of large size to accommodate the necessary current to power the series-connected heaters. Lower-powered, more comfortable, and less expensive alternatives were desired.
More recently, vehicle seat heating devices have typically electrically connected the seat bottom portion and backrest portion heating elements in parallel, each with a respective controller. The controllers may be individually adjusted by the occupant to control the amount of physiologically-sensed heating provided to the seat bottom portion and backrest portion. For example, German Patent Publication No. DE 3732841 (the '841 Publication) discloses a device for heating a vehicle seat that includes parallel-connected first and second control units that are coupled to the seat bottom portion and backrest portion heating elements, respectively, and which are separately controlled and powered through a dash-mounted occupant switch. The occupant switch is connected to the vehicle's electrical system, which includes a source of direct current (e.g., the vehicle battery), and allows each of the respective heating elements to be individually actuated and the amount of current directed therethrough to be individually adjusted with a potentiometer. The occupant switch thus allows a passenger to select desired temperature settings for the device's seat bottom portion and backrest portion heaters, each respective selection having an occupant switch output voltage of corresponding temperature selection value.
In the '841 Publication, power for the first and second heating elements is received from the occupant switch and is directed through first and second control units, respectively. Each control unit includes an operational amplifier and a unipolar or field effect transistor (FET), with the operational amplifier controlling the gate voltage of its respective FET based on the respective temperature selection value and a respective temperature measurement output signal received from a temperature sensor disposed in the seat bottom portion or backrest portion. Each temperature sensor is positioned at a predetermined distance from, and is in thermal communication with, its respective heating element for measuring its thermal output and generating a corresponding temperature measurement value. Thus, the thermal input to each temperature sensor is influenced by the amount of current being directed through its associated heating element, and provides the respective control unit with feedback (i.e., the temperature measurement value) that is compared with the temperature selection value received from the occupant switch. The comparison of temperature selection and measurement values by the operational amplifier is determinative of the amount of current to be allowed to pass through the FET and to its heating element.
In the '841 Publication, the occupant switch mounted in the dash is provided with sufficient electrical current to accommodate powering both the first and second heaters simultaneously. The conductor feeding power to the occupant switch from the vehicle's source of direct current must therefore be sized appropriately large. Moreover, each of the first and second heating elements has its own temperature sensor for providing feedback to its respective control unit. Powering and controlling each of the first and second heating elements therefore requires three conductors extending between the seat bottom portion or backrest portion and its associated first or second control unit. Each of the control units requires three conductors extending between it and the occupant switch. Those of ordinary skill in the art will also appreciate that distances between the switch and/or control unit(s) and/or its respective heating elements bear on the size, material, cost, and failure risk of conductors extending therebetween.
Allowing the individual heating elements of seat heating devices to operate simultaneously, as disclosed in the '841 Publication, can significantly tax the vehicle's electrical system. Further, the consequent size of conductors and the complexity of wiring harnesses for such devices can undesirably affect device cost and reliability. Moreover, the use of multiple feedback elements such as the above-mentioned plurality of temperature sensors can present multiple opportunities for system failure that can result in uncontrolled activation of a heating element, which can result in occupant discomfort or further system failures due to overheating, or require continuous control adjustment by the occupant.
In the past, addressing such risks of failure typically entailed providing the system with redundancies or large design factors of safety that further adversely affect costs and complexity, or which may noticeably compromise heater performance.
An opportunity exists, therefore, to improve upon existing devices for heating a vehicle seat by addressing such concerns.